A 40-foot-tall section of the tower’s glass outer wall at a California testing lab is being pelted with rain, hammered with sustained winds of 74 mph, exposed to extreme heat and cold, and shaken by ground-shaking vibrations.

The mock-up of a corner section of three upper floors of the tower includes 24 glass panels like those that will one day make up the tower’s outer skin – 1 million square feet of glass that will rise 1,368 feet when completed in 2011. An antenna will make the total height a symbolic 1,776 feet.

Technicians at Construction Consulting Laboratory West in Ontario, Calif., are putting the mock-up through two weeks of testing that covers 26 structural and meteorological conditions.

An enormous Curtiss-Wright airplane engine churns up the wind power aimed at the glass wall, which will face the whipping winds off New York Harbor when it is constructed at Ground Zero.

The Freedom Tower, designed by architect David Childs of Skidmore, Owings & Merrill, will have a skin made of 4-inch-thick glass panels that will stretch panel to panel, with no structural elements between the 13-foot-tall panes of glass.